usb/dwc3: Only force USB2 based on the PHY register and IP version

We shouldn't force USB2 only based on if we have an external PHY.
The internal PHY register tell us what link speed we can acheive
and we need to force USB2 only if it cannot do USB3.
This is only available after revision 0x290A of the dwc3 IP.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D37394
Fixed: 1331c0f44b6a ("Add support for RockChip RK356X to DWC3 driver.")
Sponsored by: Beckhoff Automation GmbH & Co. KG